Vast lava flows and boulder fields, such as Syrtis Major, usually appear as dark markings on the surface. However, their visibility varies because winds carry dust across them, obscuring them in whole or in part. Blowing dust can also hide smaller markings or make them difficult to identify.
Sometimes you will see no markings at all for days or weeks on end, thanks to one of the global dust storms to which Mars is prone. These storms occur most often during the southern hemisphere's spring and summer.
The last global dust storm was in 1982, but localized ones occurred in 1988, 1990, and
1992, and observers should always be alert to their onset.